18V bipolar power supply for LM3886 amp

[petemoore]

  A good preamp has a sensative input, the layout on the breadboard...I'd say there'd have to be a problem somewhere if it didn't hum .
  Electronic artwork !

[Brymus]

Thats pretty cool looking for what you had to work with.
If I were to guess,I would say your wiring>you have alot of wires running to and fro.
That transformer for your heaters puts out a pretty good flux field,proximity of spkr jack.
Also you could use shielded wire to your input grid.
Search gain clone amps,thats basically what yours is and look at the layouts of others for ideas.
Maybe PRR or someone can give you better detailed advice on your layout.
Congrats its almost done.

I will listen to the sound clips you sent me after the baby wakes up 

[R.G.]

@El: Let me interject another calming note.

Tube amps are notorious for having some residual hum. One probably apocryphal tale has it that Bob Carver accepted a challenge to make one  of his power amps sound indistinguishable from a tube power amp, and did it by carefully analyzing and reproducing the frequency response and damping of the comparison tube amp, and then introducing just a touch of residual hum. He supposedly won the bet.

I've just recently spent several weeks in an ongoing advice/debug set of posts with a fellow over the hum in a factory-made AC30 in another forum.

De-humming a tube amp (yours qualifies because most hum comes from power supplies and preamps, not the power amp) is like the old analogy of peeling an onion. You take off a layer or two at a time, go to the next layer, and cry a lot. Don't lose heart because you still have some layers to go.

[El Heisenberg]

I will try moving the output jack and using shielded wire for the input. As of now, the tin foil tube shield is keeping out a TON of noise. take it off and it goes 'BRMUUUUUUHHHHHHHHHH!".  As for wiring, I did use more than I would normally, but I read to use seperate wires for this and that. If I woulda just done it, I would have tied all grounds right to the capacitor 0v lead. I would have put the capacitors right onto the bridge rectifier instead of using seperate wires. I don't want to know what it'd be like if I'd have done it like that tho.

But all the wires going everywhere are really ground wires. If I had more than one color wire I'd be easier to tell. I thought it wouldn't matter where I put ground wires since they'd be going to the star ground. I tried to keep the other connections short.

The only noise I get is from turning the bass up past 8 o clock. at 9 or 10 o clock it gets big. The md knob doesn't put in too much anymore.


With this noise it's still very usable and cool. But it'd be way way cooler if it were really quiet.

[phector2004]

that's a really cool VCR amp!

just a thought, maybe the gurus can comment on this: I accidentally bought 2-conductor shielded wire. could something like this further reduce hum by shielding the heater wires, wires going to the switch, etc? (any significant amount more than just braiding, at least?)

[El Heisenberg]

HEY! Do these spring things act just like tube shields??!?  I think I could just make something sorta like those.

[El Heisenberg]

just a thought, maybe the gurus can comment on this: I accidentally bought 2-conductor shielded wire. could something like this further reduce hum by shielding the heater wires, wires going to the switch, etc? (any significant amount more than just braiding, at least?)

Yea and I was kinda wondering if were I put the twisted AC heater wires was ok or not.

[El Heisenberg]

All the thin red wire you see in the pics is solid copper. Should I have used the braided wire??

It was all I had on hand and I didn't wanna spend money on wire.

[El Heisenberg]

I think I'm kicking a dead horse here. Maybe the tube preamp is so sensitive, it's jsut picking up crazy hum when the bass is turned up and especially when the mid is turned up with the bass. Other than that, I'm at full blast and all I have is some weird intermitent hissing. It's surprisingly quiet for how loud it is. This is a nice chip. I wonder if I really shoulda got a hotter transformer to make it really loud. Maybe I'll do that later.

But yea the dead horse...When I turn the volume control down the hum goes away completely (with the tin foil tube shield). I tried shielding every signal wire in that area. And there was no differece. PRR originally gave me his drawing with a 250k volume pot. I put in in a 1 M every time I built it cos I liked the extra volume. It was never an issue until I was amplifying the preamp so much!!! So I'm letting it be. Everyone said hum is just part of a tube amp. And it's not that loud. I'd like to hear the hum on a real tube amp. I just don't feel like going to guitar stored and plugging guitars in and not really playing anything.

I added my digital reverb circuit in all neatly. I did it with another little 12.6v 300mA transformer and a 12v reg. Now the thing sounds amazing. I wanted to use a DPDT switch to change the tonestack to the Umble type that I saw on runoffgroove.com. I thought it should work. I thought I did everything right but I couldn't get it to work. I'm prolly just real tired from messing with it all. Yea see I'm rambling.





Tho, i'd really like to know if those spring thingies pictured above shield the tubes like a normal tube cover or if they're just there cos of just because because.

[R.G.]

I think I'm kicking a dead horse here. Maybe the tube preamp is so sensitive, it's jsut picking up crazy hum when the bass is turned up and especially when the mid is turned up with the bass.

People who have only done solid state are usually astonished at the hum pickup abilities of tube equipment. This is partly because tubes work at (in general) much higher impedances, and are prone to capacitive pickup, and partly because tube circuits come from a time when modern techniques for power supply immunity were not used.  That LM3886 you're using pretty much does not care if there is a few volts of ripple on its power supply as long as its ground is clean.

Other than that, I'm at full blast and all I have is some weird intermitent hissing.

That would be parasitic oscillation being kicked off. Capacitive feedback is getting something going. An "angry" sounding hiss in audio is often the symptom of RF oscillation back-modulating the audio path. Thermal hiss is more polite sounding. 

It's surprisingly quiet for how loud it is. This is a nice chip. I wonder if I really shoulda got a hotter transformer to make it really loud. Maybe I'll do that later.

You will be surprised how little going from where you are to the highest power supply voltage you can put on that chip helps with loudness. You're well out of the region where small power increases make the speakers sound much louder. It will, for instance, take ten times the amplifier power to make the perceived volume be twice as loud.

But yea the dead horse...When I turn the volume control down the hum goes away completely (with the tin foil tube shield).

And this tells us that the hum is being picked up and passed on before the volume control, right?

I tried shielding every signal wire in that area. And there was no differece. PRR originally gave me his drawing with a 250k volume pot. I put in in a 1 M every time I built it cos I liked the extra volume. It was never an issue until I was amplifying the preamp so much!!! So I'm letting it be. Everyone said hum is just part of a tube amp.

Hum is an ever-present threat. It is possible to get an almost hum-free tube amp. But you have to not only know what you're doing, you have to work hard at it. There are many ways for hum to get in.

I'd like to hear the hum on a real tube amp. I just don't feel like going to guitar stored and plugging guitars in and not really playing anything.

Why not? I'm sure the store people would love someone who just wants to audition hum and hiss levels instead of yet another rendition of "Stairway to Heaven".

Tho, i'd really like to know if those spring thingies pictured above shield the tubes like a normal tube cover or if they're just there cos of just because because.

They're there to hold the tubes firmly in place. Retainers. No significant shielding.

[El Heisenberg]

Ah cool. Thanks, R.G.!

Looks like I'm done!! There's still hum at the input even when nothing is plugged in. I'll try disconnecting stuff and see where it goes. I still don't know enough to get rid of it. I think I'll come back later and be able to fix it.

[El Heisenberg]

Eh, one more question. Would using a 12.6v 3A transformer for the tube filaments be better than using a 12.6v 1.2A transformer? I have both now and was wondering if it's worth the effort to replace it.

[Brymus]

IIRC  the 12**7 series only draws 150 ma at 12V and 300ma at 6.3V,so you have more than enough for one or even two tubes (4 triodes total)
Also solid or stranded wire doesnt make a diff in your application (some would disagree)
You might get less noise running your heaters on DC (again some would disagree) in my own amps I find it to be noticably quieter with DC heaters.
I also use shielded wire for the signal path except for short runs between tubes.
Keep your wires as close to the chassis as possible(you probably did already),and read up at Valve Wizard or any tube site, GEOFX is good too, on what wires dont play well next to each other ,I forget off the top of my head.
Looking at some layouts of tube pre amps might be of some help with that as well.

[El Heisenberg]

thanks, brymus.


I have one more question because I just popped in a tube that I've never tried in that amp. It's a JJ electronics 12AU7 and there's no need for a tube shield. I took it off and it's totally fine. I don't get it. I need a tube shield for my other two tubes, but not this one.

[R.G.]

The phrase "a rose is a rose is a rose" does not apply to tubes. All tubes are not equal, and there is no valid set of industry standards getting tubes to be, well, standard.

I would be surprised if multiple different tubes from different places - or even the same place from different batches - all sounded and acted the same. There was a massive engineering effort expended to get all tubes of a given type to be the same back in the Golden Age. That effort no longer exists. To the electronics world, tubes are an atavistic curiosity.

[El Heisenberg]

I knew tubes varied from brand to brand, and since I know it's old tech, I expected differences from peice to peice. But I didn't expect such a drastic change.

I thought I needed the tube shield because the transformers or something were giving off noise that the tube was picking up. When I covered it with tin foil it behaved. Witht his tube, I just pug it in and play. The tin foil does nothing. There's no loud hum without it. Do some tubes have some sort of internal shielding or a better layout or something? All the tubes I have do sound different. This tube is my least favorite sounding, but was most expensive. It's the only one that's totally quiet tho.

[R.G.]

More likely the heater is leaking in the one with the bad hum. Just a guess. Mark that one as "Hums a lot" and put it back in the box as a spare if the others all burn out and you don't have any other replacement.

[dschwartz]

all this hum subject makes me remember my attempts to build a couple of tube driver clones (years ago)..i just couldnt get rid of the hum....too many sources of noise and limited patience made me stop my attempts..

strangely, my latter 4-triode tube preamp designs were pretty noise-free (but not without extensive learning about grounding, shielding, and layout)

maybe your layout is wrong in some way..if i were you, i´d discard the tube and have fun designing something SS..for example using joe davison "diode compression opamp" will get you even closer tu a tube amb than an actual triode there

[El Heisenberg]

Well, I can live with this hum until I build the next amp. Every amp I build sounds better than the last, and this is no different. It's also crazy louder.

I couldn't leave it alone. I tried to add a switch to switch from the fender tone stack to the Umble tone stack on Runoffgroove.com. I thought I had it with a DPDT switch. Maybe my connections were bad or putting a 12k resistor across a 100k pot to make it around 10k doesn't work.


I added a tremolo too. A really really simply one.



The MPF102 is by the input and there's a SPDT switch to lift the depth pot and disengage the tremolo.

I was going to try something like what the vibracaster is, but I didn't want the tremolo after the reverb, which is before the tube preamp.
I wanted to put the reverb after the preamp, but I was afraid there would be problems.

[petemoore]

  The Piaa stereo preamp k100 [on sale ] should show up here, hopefull 'build itself' and be ready for the LM3886's application.
  More than happy to describe how it goes..
  Preamp outside the amp area [in separate enclosure].
  The times I've DIY'd preamps:
  Made as easy to be fiddled with, until a replacement is the preamp or performance has been optimized, can't be outclassed [easily.
  Racing preamps is fun business, I have a Presonus TubePre...a 'top racing champ'..nice as a standard [comes in very handy with the Beta58 mic/98% of all other 'patch through / preamp / cable conversion / matching' needs.
  Great preamps are inherently sensative.